华南前汛期锋面对流系统和暖区对流系统的多尺度特征和集合预报敏感性对比分析

Analysis of Multiscale Features and Ensemble Forecast Sensitivity for MCSs in Front-Zone and Warm Sector During Pre-Summer Rainy Season in South China

2018年5月7日华南地区受锋面中尺度对流系统和暖区对流系统影响,出现多条中尺度雨带。其中锋面对流系统形成降雨区范围较广,雨量分布不均;在锋前30~200 km暖区内,多个离散的短生命史β中尺度对流系统形成范围较小的中尺度雨带;而在华南沿海地区中尺度线状对流长度超过300 km,稳定维持时间超过12 h,形成局地300 mm以上的沿海强降雨带。雷达回波分析表明华南地区的锋面对流系统、暖区对流系统均以低质心型对流单体为主,其中锋面对流单体35 dBz回波顶高平均为5.5 km,暖区对流系统35 dBz回波顶高平均为4.7 km。利用ERA5再分析资料诊断降水效率表明,锋面系统降水效率平均在10%~15%,暖区对流系统的降水效率波动明显,瞬时降水效率可超过90%。此次降雨过程中雨滴谱分析表明,小粒子直径、高雨滴数密度的暖云降水特征突出,沿海暖区对流系统在各个降水强度量级上都具有更大的粒子直径和数浓度,因此降水效率较高。预报检验表明主流业务数值模式对于暖区对流性降水预报能力有限,欧洲中心再预报改善了暖区对流性降水离散度分布,中尺度区域数值模式能够反映锋面对流和暖区对流的基本特征,但在沿海暖区对流系统的强度、组织上仍然有偏差。比较锋面降水和暖区降水的集合预报敏感性表明,锋面降水对于锋前低压槽、低空急流等天气系统强迫具有较高预报敏感性,而沿海暖区降水对于上游入流区不稳定能量分布具有更显著的敏感性。

Severe rainstorms struck South China on 7 May 2018,which were related to front-driven mesoscale convective system(MCS)and quasi-stationary MCS in warm zone.South-moving frontal MCS led to wide range of un-uniform precipitation,and warm-sector MCSs with short life time induced multiple mesoscale rainbands within the range of 30-200 km south to surface front.Quasi-stationary line-type MCS along the coast of South China grew up to 300 km in length,sustained more than 12 h and induced extremely severe rainfall beyond 300 mm.All types of MCS showed the low-echo-centroid structure on the vertical section of reflectivity,and the averaged 35 dBz echo-top was at the height of 5.5 km for frontal MCS cells,compared with 4.7 km for convective cells in warm sector.Analysis of raindrop size distribution revealed that MCS in coastal zone was with larger raindrop diameter and higher nuclei concentration compared with frontal MCS.Averaged large-scale precipitation efficiency of frontal convective system was at about 10%-15%according to ERA5 reanalysis,while the precipitation efficiency of MCS in warm zone could increase instantaneously more than 90%.Operational numerical models showed limited predictivity for convective precipitation in warm zone,while EC-Reforecasts showed improved performance on ensemble spread for the convective precipitation near South China coast.Mesoscale model’s forecast showed basic patterns of frontal MCS and warm-sector MCS,however obvious bias existed in the organization and intensity of MCS along coastal zone.Ensemble sensitivity analysis indicated that frontal MCS showed high sensitivity to synoptic forcing related to low pressure trough and low-level jet intensity,while convection in warm sector showed high sensitivity to CAPE in upstream environment.